Process and apparatus for electrostatic application of liquids or powders on substances or objects

ABSTRACT

The present invention relates to a process and an apparatus for electrostatic application of liquids or powders on many different substances and objects, by means of a cone diffusing the substances or objects to be coated, in free fall continuously along the internal wall of a tube where a high voltage positive corona zone is produced, while the liquid or powder is prayed from a centrifugal atomizer, by which a high voltage negative corona zone is produced, so that the liquid or powder is charged negatively and is attracted by the substances or objects charged positively and sliding on the wall of the tube.

BACKGROUND OF THE INVENTION

The present invention relates to a process and an apparatus for theelectrostatic application of liquids or powders on many differentsubstances, particles and objects, with truly excellent results whichcannot be obtained in any way with the presently known processes.

The invention uses the well established principle of the electrostaticattraction between materials, whether liquid or solid, when they receivehigh electrical charges of opposite sign. Electrostatic charge effectshave been used for a number of years in the application of paint tometal objects of a considerable size, such as washing machines,refrigerators, motorcars, or in photocopying machines.

With the process and the apparatus of the present invention it ispossible to apply liquid or powder coating on many different nonmetallic natural, artificial or synthetic materials, such as powders,pellets, tablets, seeds, grain and similar particles and substances oreven objects and material of small individual size.

SUMMARY OF THE INVENTION

The process according to the present invention is characterized by thefact of comprising the steps of producing a substantially continuouscircular curtain of the product to be coated, causing it to pass on astationary diffusion means, then giving it a high positive electricalcharge by passing it through a positive corona zone, whilst in freefall; diffusing the coating liquid or powder through a centrifugaldiffuser means where it is given a high negative electrical charge bypassing it through a similar but negative corona zone, so that theparticles of the product to be coated whilst in free fall, are totallyand uniformly envelopped by the liquid or powder coating particles,which are attracted thereto because of the opposite sign of theircharges.

The apparatus according to the present invention is characterized by thefact of comprising a feeding device for the product to be coated, havingat its end a stationary diffusion device producing a substantiallycontinuous circular curtain of said product whilst in free fall on thewalls of a non conductive tube, where a high voltage positive electrodeis placed, producing a positive corona zone, positively charging theparticles of the producto be coated; a central feeding duct for theliquid or powder coating particles, having at its end a centrifugaldiffuser member provided with a high voltage negative electrode,producing a negative corona zone, negatively charging the particles ofthe liquid or powder coating; so that the particles of the productwhilst in free fall, are totally and uniformly evenlopped by the liquidor powder coating particles, which are attracted thereto because of theopposite sign of their charges.

When the product is to be coated with a liquid, the centrifugal diffuserdevice is a centrifugal atomiser fed by a micro pump or similarcalibrated, accurate and controlled feeding device for the spray liquid.

If it is required to coat the product with a powder then the liquidcentrifugal atomiser is replaced by either a powder gun fed from afluidiser or similar powder system having a 360° diffuser horn, or coneshape fitted concentrically to the powder flow, or again using thecentrifugal atomiser, but with the liquid feed system replaced by asuitable powder projection system.

The throat of the stationary diffusion device for the product to becoated may be adjusted to regulate the quantity of product to bediffused in free fall.

It is also possible to charge the powder with a friction processingdevice (frictional electricity) or by an electro gas dynamics system.

The process and the apparatus according to the present invention havethe following considerable advantages over conventional coating methods:

(1) The product to be coated does not need to be earthed, therefore nonconductive materials can be used.

(2) Since coating takes place in free fall it is possible to producemore continuous films than by other systems, and practically uniform andtotal coatings.

(3) By using a curtain of product totally surrounding the delivery headfor the coating liquid of powder, losses of such a spray liquid orpowder are totally eliminated.

(4) Unlike ordinary centrifugal coaters, this system of counter chargingproduces mutual directional attraction between the product to be coatedand the coating spray. This then eliminates variations caused bydiscontinuity in the circular product curtain and is instrumental ineliminating normal coating spray losses, which can otherwise occur.

(5) The operation of the plant is absolutely continuous and by linkingthe feed pump control for the coating spray to the product flow by meansof a computer unit, a constant ratio of coating spray to product can bemaintained.

This is most important in the addition of vitamins and antibiotics andin the addition of Amines to Ammonium Nitrate fertiliser pellets, whichare explosive above 0.7%.

(6) The electrical energy consumption is less than 200 Watts for athroughput of 200 Kgs per Minute, and therefore the system of theinvention is very cost effective compared with conventional rublers anddrum coaters using up to 50 Horse Power for similar outputs.

BRIEF DESCRIPTION OF THE DRAWING

Objects, features and advantages of the invention will be apparent andclear from the following description of an embodiment of the apparatusaccording to the present invention, which is given as a non limitingexample only and with reference to the accompanying illustrativedrawings, whose single FIGURE shows said apparatus in a ratherdiagrammatic form.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference now to said single FIGURE of the accompanying drawing,the apparatus according to the present invention substantially comprisesan outer frame 1, at the top center of which a hopper 2 for the productto be coated is fixed, said product being deviated and diffused at itsbottom by a stationary diffusion cone 3 so as to produce a continuouscurtain of free falling product along the walls of the internal nonconductive tube 4, fixed to the outer frame 1 by insulators 5. On theinternal tube 4 a high voltage positive electrode 6 is arranged,producing a positive corona zone, giving to the particles of the productto be coated a high positive charge up to 90-100 kV. The polarity of theelectrodes 6 and 11 may also be inverted.

At the center of the apparatus, the axis on which the stationarydiffusion cone 3 is mounted, consists of a pipe 7 for feeding thecoating liquid or powder, coming from a feeding pump 8, said pipe 7being also provided with the negative power line 9. The lower end ofpipe 7 is formed by the rotary atomizer head 10, spraying the liquid orpowder on the curtain of product to be coated, whilst in free fall alongthe wall of the internal tube 4. Atomizer 10 is also provided with thehigh voltage negative electrode 11, producing a negative corona zone,giving to the coating liquid or powder a high negative charge, also upto 90-100 kV.

It is therefore clear that the particles of coating liquid or powder,negatively charged are attracted by the positively charged product andcoat it totally and without any waste, the so coated finished productbeing discharged by the bottom mouth 12 of the internal tube 4 on asuitable carrying and/or discharging device (not shown), which could befor instance an endless conveyor belt or any other device, mainlydesigned according to the kind of product to be processed.

The width of the throat or nozzle for the diffusion of the product to becoated, formed by the distance between the lower end of the hopper 2 andthe wall of the stationary diffusion cone 3, may be adjusted in order tochange the flow rate of product to be coated which is introduced intothe processing chamber, said adjustment being diagrammatically shown indotted lines on the drawing.

As merely illustrative and non limiting examples only, the process andthe apparatus according to the present invention have been proved by thefollowing successful experiments:

(a) The addition of 0.2% of oil containing vitamins to skim milk powderat the rate of 1.5 Tonnes/Hour. Analysis on 2 gm samples showed lessthan 0.5% variation in vitamin content.

(b) The addition of 2 Kgs per Tonne of Amine Solution to AmmoniumNitrate Fertiliser pellets of 1-2 mm diameter at the rate of 200Kgs/Minute.

(c) The addition of Antifungicides and Bird Repellents to seedsproducing a 30% improvement in film continuity over Non-ElectrostaticFree Fall systems.

(d) The addition of up to 5% fat with or without the further inclusionof antibiotics and vitamins to pig weaner pellets at a rate of 1Tonne/Hour.

(e) Addition of flavours such as Peppermint Oil and Organge Oil to sugarand to compressed sugar confectionary.

(f) The addition of wetting agents to very fine powders of bulk densityof 0.3 Kgs/Liter as a first step in the formation of slurries.

From the foregoing it is therefore clear the great versatility of theprocess and the apparatus according to the present invention and theexceptional quality of the coated products so obtained, and it is to beunderstood that many variations, modifications, additions and/orsubstitutions of elements may be resorted thereto, without alteratinghowever its spirit nor departing from its scope of protection, as it isdefined in the appended claims.

We claim:
 1. A method for the electrostatic coating of objects with aliquid or powder material comprising the steps of:(a) producing asubstantially continuous free-falling circular curtain of the objects tobe coated; (b) charging said objects with a high electrical charge of afirst polarity by passing said objects through a first corona dischargezone while said objects are in free-fall; (c) diffusing said coatingmaterial by passing it through a centrifugal diffusing device anddirecting said coating material toward said curtain of objects to becoated; and (d) charging said coating material with a high electricalcharge of a second polarity opposite to said first polarity by passingsaid material through a second corona discharge zone,whereby saidcoating material is electrostatically attracted to said objects tototally and uniformly coat said objects.
 2. Process according to claim1, characterized by the fact that a constant ratio between said objectsto be coated and said coating material is maintained by an electroniccontrol regulating the feed of coating material as a function of theflow of said objects.
 3. Apparatus for the electrostatic coating ofproducts with a liquid or powder material comprising a feeding means forintroducing the product to be coated, a non-conductive tube comprising awall surrounding the end of said feeding means, said feeding meanshaving first diffusion means at the end thereof for producing asubstantially continuous free-falling circular curtain of the productagainst the walls of said tube, a high voltage electrode adjacent thewall of said tube for producing a corona discharge zone of a firstpolarity to charge the product with said first polarity, a central ductextending into said tube for introducing the coating material therein,second centrifugal diffusion means at the end of said duct for diffusingoutwardly the coating material, a high voltage electrode adjacent saidsecond diffusion means for producing a corona discharge zone of a secondpolarity opposite to said first polarity to charge the material withsaid second polarity, whereby the coating material is electrostaticallyattracted to the product to coat the product.
 4. Apparatus according toclaim 3, characterized by the fact that, when the product is to becoated with a liquid, the second centrifugal diffusion means is acentrifugal atomiser fed by a micro pump or similar calibrated, accurateand controlled feeding device for the spray liquid.
 5. Apparatusaccording to claim 3, characterized by the fact that, when the productis to be coated with a powder, the second centrifugal diffusion means,is a powder gun fed from a fluidiser or similar powder feeding systemhaving a 360° diffuser horn.
 6. Apparatus according to claim 3,characterized by the fact that, when the product is to be coated with apowder, the second centrifugal diffusion means is a cone shape fittedconcentrically to the powder flow.
 7. Apparatus according to claim 3,characterized by the fact that, when the product is to be coated with apowder, the second centrifugal diffusion means is a centrifugal atomiserprovided with a suitable powder projection system.
 8. Apparatusaccording to any one of claims 3-7, characterized by the fact that thethroat of the first diffusion means for the product to be coated may beadjusted to regulate the quantity of product to be diffused in freefall.
 9. Apparatus according to any one of claims 5 through 7,characterized by the fact that the powder is electrically charged by afriction processing device or by an electro gas dynamics system.